This application claims the priority of German Patent Application DE 10235158.9, filed Aug. 1, 2002, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a circuit arrangement for adapting the characteristic wave impedance at line ends of a data bus in a vehicle.
The current state of the art for interconnection of computers and computer systems includes high-speed data transmission systems that are connected to one another (normally in a star-shaped configuration, by means of switching systems or gateways) via screened lines that are balanced with respect to ground. Data rates of up to 100 Mbit/sec can be implemented by means of copper lines. Even higher data transmission rates are possible using cables made of glass fiber.
When these technologies are applied to data bus systems in vehicles, their electromagnetic compatibility (EMC) must be taken into account in order to fulfil the EMC regulations for motor vehicles (for example ISO 7637, DIN 40 839). To do this, the control units must be immune to external electromagnetic influences. In addition, the electromagnetic emissions from data bus systems must be minimized.
Data bus systems or cable networks in vehicles are composed of connecting lines that couple a multiplicity of different data terminals to one another. Such data terminals include, for example, the various electrical and electronic systems such as ignition system, electronic injection system, anti-lock brake system, airbag, car radio, car telephone, sensors, actuators, etc., which are installed in the vehicle and interconnected via a data bus system. The data terminals have a transceiver unit for connecting to the data bus system. Depending on the type of connection, the connecting lines to the data terminals fulfill different functions, including in particular the supply of direct current, control and exchange of data.
However, in addition to the direct supply currents and useful signals which are used for this purpose, radio-frequency interference signals, which are either generated in the vehicle or radiated in from the exterior, also occur in such cable networks. In the frequency range of such radio-frequency interference signals, the cable networks exhibit a multiplicity of resonant frequencies. Given such resonant frequencies, the interference signals can reach high voltage and current values, which can lead to considerable interference or damage in sensitive and high-impedance electronic components, for example those using CMOS technology. Resonant frequencies also promote the radiation of interference signal harmonics which are generated in the vehicle.
In addition, the data terminals are generally not adapted to the common-mode properties of the vehicle network so that in all their branches there are resonances for interference signals that are generated externally. Moreover, the suppression of these interference signals is thus also made difficult.
Basically, a number of factors are responsible for propagation of electromagnetic interference in cable networks. In particular the line properties such as characteristic wave impedance of the connecting lines and the properties of the line terminations as well as of the data line reactor as a component play an important role here.
German Patent Document DE 196 36 816 C2 discloses an arrangement for reducing radio-frequency interference in cable networks for supplying current and control signals in vehicles, in which ferrite reactors are installed in individual connecting lines and provide a radio-frequency impedance that is high in comparison with the characteristic wave impedance of the lines. This measure gives rise to a passive characteristic wave impedance termination, and the load is decoupled from the line network.
High-speed data transmission systems for data transmission rates of at least 10 Mbit/sec are new. They also generate common-mode spectra whose maximum frequencies may rise up to 1 GHz, making it more difficult to combat them than in the past. The reactors which have been used hitherto are sector-wound, with a scatter of 1-2 μH. The useful signal is distorted by the abovementioned leakage inductance to such an extent that it can no longer be used for the transmission of data.
The article “Drosseln sichern EMV auf Kfz-Bussystemen [Reactors ensure EMC on motor vehicle bus systems]” (K. Marth, Siemens Components 5/93, pages 172-174) discloses various data line reactors for CAN bus systems which ensure fault-free functioning of the CAN bus system and increase the insertion loss in the common mode by 10 dB in comparison with a standard reactor.
One object of the invention is to provide a circuit arrangement that optimizes the adaptation of characteristic wave impedance at the cable ends in order to suppress externally generated in common-mode interference voltages.
This and other objects and advantages are achieved by the circuit arrangement according to the invention, in which an adaptation resistor network is arranged between the reactors and the respective line ends of the data bus lines. The adaptation resistor network connects the line ends of the data bus lines to ground, via a capacitor.
Transceiver units of data terminals are data transmission drivers. These units form the electrical connection between the data terminal and the cable, reactors being connected between the transmitter component and receiver component of the transceiver unit to filter interference.
In particular, this arrangement can be used to suppress common-mode interference signals. In this respect, it is significant that the adaptation resistor network is placed directly between the reactors and the data bus line (that is, at the line end).
The circuit arrangement according to the invention also ensures that the characteristic wave impedance of the cable is adapted to the given impedance of the transceiver unit.
Terminating the data bus lines by means of the adaptation resistor network prevents the open data bus line from being made to resonate to a high degree, which also reduces the reflection.
The circuit arrangement according to the invention also has the advantage that the voltage at the line end is damped by the reactors upstream of the transceiver unit.
In the adaptation resistor network, the stabilizing resistors are preferably connected to ground via a common base capacitor, making it possible to dispense with a second capacitor (that is, a further component).
The values of the stabilizing resistors of the adaptation resistor network are ideally determined for adapting the source resistance of the transceiver unit to the characteristic wave impedance of the data bus lines in differential-mode voltage. As a result, common-mode interference signals and differential-mode interference signals are simultaneously suppressed to an optimum degree.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.